1. Field of the Invention
The present invention relates to an ink jet recording apparatus to output information such as characters and images onto recording media in information processing systems including copying machines, facsimiles, printers, word processors and personal computers. This invention also relates to a cap mechanism used in such an ink jet recording apparatus.
2. Description of the Related Art
Conventional recording apparatuses using recording media such as paper, cloth, plastic sheets and OHP sheets (hereinafter referred to simply as recording paper) have been proposed with various types of recording heads to perform recording methods such as wire dot recording, thermal recording, heat transfer recording, and ink jet recording.
Among these recording methods, the ink jet method is one of low-noise non-impact methods to discharge ink directly onto the recording paper. The ink jet method is classified largely into a continuous method (including a charged particle control method and a spray method) and an on-demand method (including a piezo method, a spark method and a thermo-applied method).
The continuous method delivers ink continuously and gives electric charges only to required ink droplets so that the charged ink droplets adhere to a surface of the recording paper, with the remaining uncharged ink droplets discarded. On the contrary, the on-demand method delivers ink only when printing is demanded so that ink is not wasted and that the interior of the recording apparatus is not fouled. Further, because the on-demand method starts and stops the discharge of ink, its response frequency is generally low when compared with that of the continuous method. To cope with this problem, the on-demand method has an increased number of nozzles to realize high-speed printing.
Many of the recording apparatuses on the market are therefore of the on-demand type. Because they can perform high-density and high-speed recording, the recording apparatuses with such ink jet recording heads have found a wide range of applications in the market for output means of information systems, which include printers as output terminals of copying machines, facsimiles, electronic typewriters, word processors and workstations and handy or portable printers for personal computers, host computers, optical disk drives and video equipment. In these applications, the individual ink jet recording apparatuses have specific constructions suited to their particular function and use.
The ink jet recording apparatus in general comprises a carriage carrying a recording device (recording head) and an ink tank, a feed device for feeding recording paper, and a control device to control these devices. The recording head that discharges ink droplets from a plurality of nozzles is serially moved or scanned in a direction (main scan direction) perpendicular to a direction of feed (subscan direction). During non-recording intervals the recording paper is intermittently fed a distance equal to the recorded width. This recording method delivers ink onto the recording paper according to recording signals and has the advantages of low running cost and quietness, and is thus widely used. With the ink discharge openings of the recording head arranged linearly in the subscan direction, the recording head need only scan the recording paper once to perform recording of a width corresponding to the number of discharge openings. This makes for increased speed of recording operation.
In the case of a color ink jet recording apparatus, ink droplets discharged from recording heads of multiple colors are superimposed on each other to form a color image. Generally, color recording requires three or four sets of recording head and ink cartridge that correspond to three primary colors--yellow, magenta and cyan--and black. In recent years, recording apparatuses, which can form full-color images by using such recording heads of three or four colors, have been developed and put to practical use.
Further, the ink jet recording apparatus can be configured relatively easily to print a large size paper such as A1. For example, a plotter, a CAD output printer, is among the recording apparatuses already commercialized that can perform color printing on A1-size paper by using an image reader. On the other hand, a new demand for versatility in use is emerging. That is, there are growing demands for the recording of overhead projector (OHP) films used for presentations in conferences and lectures. To meet these demands, efforts are being made to develop and commercialize a recording apparatus, which can perform optimum recording on whatever kind of recording media is selected, from among a variety of recording mediums with differing ink absorbing characteristics.
In this way, demands for the ink jet recording apparatus are increasing in a wide range of industrial fields (for example, apparel industry) and there is also a call for higher quality in images produced by the recording apparatuses.
Next, an ink jet recording head (hereinafter referred to simply as a recording head) used in the ink jet recording apparatus is described.
An energy generating means to generate energy in the recording head to discharge ink includes an electro-mechanical conversion body, such as a piezo element, and an electro-thermal conversion element having a thermal resistor to heat liquid.
Among these, the recording head of a type that uses thermal energy (a surface boiling phenomenon) to discharge liquid allows the liquid discharge openings to be arranged in high density and thus can offer a high resolution of recording.
A brief description is given as to a representative ink droplet forming process performed by such a recording head.
First, when the heating resistor (heater) reaches a predetermined temperature, surface bubbles are formed covering the heater surface. The internal pressure of the bubbles is very high and pushes ink out the discharge openings. The force of inertia produced by this pushing action causes the ink to move out of the discharge openings and also in the opposite direction toward a common liquid chamber. As the ink advances, the internal pressure of the bubbles becomes negative, which, combined with the resistance of a flow path, slows down the speed of the ink inside the discharge openings. The ink pushed out of the discharge openings, because it moves faster than when it was in the discharge openings, becomes constricted and separated into droplets by the balance between the inertia force, flow path resistance, compression of bubbles and surface tension of ink. Then, simultaneously with the compression of bubbles, the capillary attraction draws ink from the common liquid chamber into the discharge openings. The recording head now waits for the next pulse.
In this way, the recording head using the electro-thermal conversion element as an energy generation means can generate bubbles in ink inside the liquid path by a drive electric pulse signal on a one-to-one correspondence basis and can also grow and contract bubbles instantly and appropriately. Because of these advantages, this type of recording head can realize a particularly responsive ink-discharge. Further, the size of the recording head can be reduced easily. Another advantage is that the recording head using the electro-thermal conversion element can be fully fabricated with a process utilizing technical advances made in the semiconductor fields in recent years and technical merits of IC and microfabrication technologies with remarkable technical advances and improved reliabilities, thus making high density fabrication easy and reducing manufacture cost.
To maintain high-quality image recording at all times, the ink jet recording apparatus generally has a head performance recovery device (hereinafter, referred as a cap mechanism). The cap mechanism is located at a position facing the home position of the recording head, for example, at one end of the travel path of the recording head. The head performance recovery device is operated under a specified condition to cap the recording head. In connection with the capping of the recording head by a cap member of the cap mechanism, an appropriate suction means (for instance, a suction pump) provided in the cap mechanism is driven to draw ink forcibly cut of the discharge openings and thereby remove viscous ink remaining in the discharge openings for recovering the ink delivery performance of the head. At the completion of the recording operation, the recording head is capped for protection. Such an ink delivery performance recovering operation is performed at time of power up, during recording head replacement or when the recording operation is not performed for more than a specified duration.
Now, the outline configuration of the conventional cap mechanism will be described in detail by referring to FIG. 1.
FIG. 1 shows a cap mechanism disposed opposite to an ink discharge surface 1a of the recording head 1 before the head performance recovery operation by forced suction is performed.
First, the outline construction of the cap mechanism will be explained.
The cap mechanism basically comprises a cap 2, a holder 3, a pressure spring 4, and a push-up base 5, and the interior of the cap 2 communicates with a pump 7 through a pipe 6.
The cap 2 is made from a resilient member such as rubber and consists of a body roughly U-shaped in cross section, an engagement surface 2a that covers the opening of the body and has a sufficient area to cover the discharge openings arranged on an ink discharge surface 1a of the opposing recording head 1, a bottom portion having an engagement surface 2b that engages with a holder 3, a suction opening formed in the bottom portion of the body, and a suction pipe 2c communicating with the suction opening. The suction pipe 2c communicates with the pump 7 through the tube 6.
The holder 3 is roughly U-shaped in cross section and has an opening in its bottom through which the suction pipe 2c is passed, and also a shaft 3a erected on its back side 3c.
The pressure spring 4 is fitted concentrically over the shaft 3a of the holder 3 between the holder 3 and the push-up base 5 to urge the holder 3 in the direction of arrow A.
The push-up base 5 is a square mount which has a hole 5a in its surface to slidably support the shaft 3a of the holder 3. The push-up base 5 is moved by a known drive means (not shown) a predetermined distance in the direction of arrow A and in the opposite direction.
Next, the operation of the cap mechanism is explained.
As the push-up base 5 is pushed up in the direction of arrow A by the drive means, the holder 3 is also pushed up in the same direction through the pressure spring 4. When the recording head 1 is located at a position opposing the engagement surface 2b of the cap 2, such a push-up action causes the engagement surface 2a of the cap 2 to engage with the ink discharge surface 1a of the recording head 1. When the push-up operation is further performed, the engagement surface 2b of the cap 2 presses against the ink discharge surface 1a. At this time, the pump 7 is operated as required to draw ink from the recording head 1--a head performance recovery operation by forced suction.
With the conventional cap mechanism, however, when the forced suction is performed, an air leakage occurs between the engagement surface 2a of the cap 2 and the ink discharge surface 1a of the recording head 1, resulting in an insufficient suction. This problem is explained in more detail by referring to FIG. 2.
FIG. 2 shows the cap mechanism in hermetic contact with the ink discharge surface 1a of the recording head 1 before the forced suction is carried out. (Components assigned the same reference numerals as those of FIG. 1 represent identical components.)
As the cap 2 is put in contact with the recording head 1 and is further pressed against it, the central portion 2d of the engagement surface 2a of the cap 2 is applied a high pressing force and undergoes elastic deformation, coming into contact with the ink discharge surface 1a of the recording head 1. The magnitude of the pressing force is not uniform over the entire cap, decreasing progressively as the distance from the point of force application increases, so that the pressing force becomes insufficient at both ends of the cap 2. As a result, end portions 2d, 2f of the cap 2 cannot undergo sufficient elastic deformation, making the air leakage likely to occur when a negative pressure is applied for the forced suction. This tendency becomes remarkable as the number of discharge openings in the recording head increases and the discharge surface becomes more elongate. A possible measure to prevent this phenomenon is to apply a strong pressing force to the extent that the end portions 2d, 2f of the cap 2 will produce a sufficiently large elastic deformation. This, however, may cause damage to the engagement surface 2a of the cap 2 and to the ink discharge surface 1a of the recording head 1 and therefore is not preferred in terms of durability.
To solve the above-mentioned problem, the following cap mechanisms have been proposed. A cap mechanism disclosed in Japanese Patent Laid-open No. HEI 5-92574 (1993), for example, attempts to make uniform the cap deformation by using a stress distributing portion and a reinforcement portion or by providing a gradient in the material strength. Making the pressure uniform over the entire cap, however, remains difficult. In another cap mechanism of Japanese Patent Laid-Open No. HEI 5-104731 (1993), the engagement surface of the cap that engages the recording head is so constructed that its central portion is lower than the end portions. This structure in principle can make the pressing force on the engagement portion of the cap contacting the recording head uniform over the entire surface and also strengthen the pressing force at the end portions. This structure, however, poses difficulty in the manufacture of components because the height of the engagement surface of the cap is varied. That is, to meet stringent requirements in terms of surface flatness and precision and to further provide variations in the height of the engagement surface of the cap is difficult to achieve in the process of making components. Furthermore, these component fabrication requirements make it difficult to offer low-priced products, a basic need of users.